COMPLEXES OF AZAPHOSPHOLES - SYNTHESIS AND STRUCTURE OF CARBONYL-(ETA(1))-2-PHOSPHAINDOLIZINE)CHROMIUM(0), BONYL-(ETA(1))-2-PHOSPHAINDOLIZINE)-MOLYBDENUM(0), AND CARBONYL-(ETA(1))-2-PHOSPHAINDOLIZINE)-TUNGSTEN(0)

The complex chemical behaviour of 2-phosphaindolizines 1 (1,3-azaphosp holo[1,5-a]pyridines) towards metal carbonyl compounds was studied. (e ta(1)-2-Phosphaindolizine)M(CO)(5) complexes 2-4 (M = Cr, Mo, W) were formed from 1 and [(THF)M(CO)(5)], the cis-L2Cr(CO)(4) complex 5f from 1f and tetracarbonyl(norbornadiene) chromium(0). The reaction of 2-ph osphaindolizines 1e, 1f, or 1g with tricarbonyl(cycloheptatriene)molyb denum(0) or tricarbonyl(mesitylene)tungsten(0) yielded sigma-complexes of the types L2M(CO)(4) or L3M(CO)(3) rather than isolable pi-complex es. In one case a strong upfield signal (delta(31)P = 6.1) was observe d with a coordination shift of Delta delta = -161.7, which is typical for pi-coordination. Prolonged reaction or work-up led, however, to di smutation yielding 1g and the iac-L3Mo(CO)(3) complex 6g. X-ray struct ure analysis of 2a indicates an increased 10 pi-delocalization compare d with 1a and a changed conformation of the acyl substituent. The infl uence of substituents and metals on the P-31 and C-13 complexation che mical shifts and coupling constants is discussed.